Te pongo en negrita la corrección:
<html>
<head>
<title>Login</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<style fprolloverstyle>A:hover {color: #FFFFFF}
</style>
<script>
function enviar(){
opener.location.href = "pagina que logeará.asp"
LOGIN.submit();
this.close();}
</script>
</head>
<body bgcolor="#B7B7FF" text="#000000" leftmargin="0" topmargin="0" marginwidth="0" marginheight="0" link="#8484FF" vlink="#8484FF">
<table width="100%" border="0" cellspacing="0" cellpadding="0" height="100%">
<tr>
<td align="center" valign="middle">
<form name="LOGIN" method="POST" action="/login.asp">
<table border="0" cellspacing="5" cellpadding="5">
<tr align="left" valign="middle">
<td align="right"><b>
<font face="Verdana" size="2" color="#8484FF">User
Id</font></b></td>
<td>
<input type="text" name="U_ID" size="15">
</td>
</tr>
<tr align="left" valign="middle">
<td align="right"><b>
<font face="Verdana" size="2" color="#8484FF">Password</font></b></td>
<td>
<input type="password" name="U_PASSWORD" size="15" value="">
</td>
</tr>
<tr align="left" valign="middle">
<td align="left">
<font color="#0000FF" face="Verdana" style="font-size: 8pt"><a href="register.asp">New
user? Click here!</a></font></td>
<td>
<input type="button" name="Submit" onClick="enviar()" value="Login">
</td>
</tr>
</table>
</form>
</td>
</tr>
</table>
</body>
</html>
Saludos!